Field of the Invention
The present invention is related generally to methods and apparatus for measuring distances to a surface without contacting that surface, and more particularly to non-contact methods and apparatus for measuring curvature of a molded glass surface immediately after such surface has been formed.
Statement of the Prior Art
In order to monitor and control the manufacture of molded glass television panels, it becomes necessary to periodically measure certain parameters of the panels such as their inside surface curvature. These measurements have in the past been accomplished typically by contacting sensors such as linear variable differential transformers, i.e., an "LVDT". One major drawback that is associated with such known measuring methods and apparatus, however, is that these contacting sensors can cause irreparable damage due to mechanical and/or thermal shock of the freshly-molded glass panels. Within the television panel manufacturing industry, therefore, it has become desirable to provide an alternative, method and apparatus for measuring those certain parameters of the panels.
Various implementations of a known technique referred to as triangulation ranging have been proposed. In most such implementations, as is illustrated in FIG. 1, a source of light 10, typically a laser, laser diode, or light emitting diode, is used in conjunction with beam shaping optics to project a light spot L onto the object surface S for which range information is desired. A receive detector, typically a position sensitive diode, linear array, or area array, 14 is offset somewhat from light source 10 and is used, in conjunctions with a lens 12, to respond to the light which is backscattered from the object. For a given distance to the object surface S, the image formed from the backscattered light will fall only on one location on detector 14.
Illustrative of such known apparatus which are used for triangulation ranging are models LC-2010 and PA-1800U/1801U optical displacement sensors manufactured by Keyence Corp. of America, Torrance, CA; model MV-300 manufactured by Perceptron of Farmington Hills, MI; models 300, 400 and 600 manufactured by Diffracto Ltd. of Troy, MI; models PRS-150, PRS-400, PRS-800 and PRS-1600 manufactured by CyberOptics Corporation of Minneapolis, MN; Optocator model 2203-503 manufactured by Selcom Selective Electronic Group; Fiberscan model FS manufactured by Optical Technologies, Inc. of Herndon, VA; model HVS-100 manufactured by Honeywell Visitronics of Englewood, CO; and model OP2 manufactured by Renishaw, Inc.
One important style of triangulation ranging which should be noted at this juncture is specular triangulation ranging. While each of the above-noted sensors can be placed at any orientation relative to the object surface, they typically work best when the light source is perpendicular to the object surface. In such position, the lens in front of the receive detector will collect diffusely scattered light, which will tend to be fairly uniform over the lens aperture. Given uniform illumination, imperfect focusing or aberrations in the lens will manifest themselves as a constant location error of the spot on the detector that may be calibrated out.
For the reasons which are detailed herein below, however, some applications require the triangulation gage to be positioned such that the lens collects the specularly reflected light. In this configuration, the input aperture of the collecting lens typically has some of its parts strongly illuminated and other parts weakly illuminated. Distribution of strong versus weak illumination varies due to minor variations in the condition and/or angle of the object surface.
Given this non-uniform, variable illumination of the lens, imperfect focussing or aberrations will manifest themselves as variable location errors of the spot on the detector, resulting in variable ranging errors given minor variations in the object's surface condition or angle. Therefore, in order to avoid imperfect focussing and aberrations, more stringent demands must be placed upon the optics of a sensor intended for use in a specular configuration. Model PRS-30 manufactured by CyberOptics Corporation of Minneapolis, MN is illustrative of an apparatus intended for specular applications.
Another important style of triangulation ranging is that in which the range information is gathered along a line or over an area of a surface by means of scanning the light source in one or two dimensions. LRS series sensors manufactured by the CyberOptics Corporation of Minneapolis, MN and model LSG-3010 manufactured by Chesapeake Laser Systems, Inc. are illustrations of known apparatus for line range sensing, while the "Three Dimensional Imaging Method and Device" as described in U.S. Pat. No. 4,627,734 is an illustration of a known apparatus for area range sensing.
During the production of molded glass television panels, however, the usage of such known apparatus for triangulation ranging is obviated for one or more of the following reasons: (1) ambient heat; (2) sensor size; and (3) electromagnetic interference (EMI) as is discussed in greater detail herein.
Immediately after its removal from the mold, the temperature of a glass television panel is typically in excess of 500.degree. C. A conventional triangulation sensor with a standoff of approximately four inches would, thus, be subjected to a great deal of ambient heat through convection and radiation. Such heat would most likely damage or unacceptably degrade the performance of all known available sensors, since those sensors generally have an operating range of from about 0.degree. C. to about 50.degree. C.
One way of solving this problem would be to surround the sensor with a cooling system such as a water jacket. Such a jacket, however, increases the overall size of the sensor, which is undesirable for the reasons discussed herein as well as its mechanical complexity. Another way to reduce the problems associated with such a high ambient heat would be to construct the sensor with a long standoff. However, for a given angle between the source of light and the detector, the size of the sensor would increase proportionally with its increasing standoff, again resulting in an undesirably large sensor.
Immediately after its removal from the mold, the surface contour of the glass television panel would be rapidly changing due to thermal shrinkage. Additionally, severe ambient vibrations are present in the manufacturing environment which cause rapid motion of the surface. Under such circumstances, one cannot reliably measure the surface contour by scanning it with a single sensor. Instead, one must employ a plurality of sensors simultaneously to "freeze" the surface of the glass television panel at a moment in time. Therefore, small sensors are desirable for such simultaneous use since they would allow more points to be measured upon the surface of the glass.
A number of cleverly miniaturized sensors are available. An example of such a sensor is the PRS series manufactured by CyberOptics Corporation of Minneapolis, MN. As is described in U.S. Pat. No. 4,733,969, for example, such PRS-type sensors incorporate a light source, imaging lens, and detector. Thereafter, signals from the detector are transmitted via ribbon cable to processing electronics that are remotely located. Remote placement of the processing electronics permits substantial reduction in the size of the sensor head. However, in reducing the sensor head size, the sensor standoff is reduced as well, thereby exacerbating the ambient heat problem noted above.
Other known small sensors are the Laserprobe models 400 and 600 manufactured by Diffracto Ltd. of Troy, MI, which have a relatively small standoff. In this respect, reference is made to U.S. Pat. No. 4,574,199. In general, small sensors having a small standoff will be especially vulnerable to heat from the glass.
It is common in processing glass to have high power electrical devices in close proximity to the manufacturing line. This is especially true in glass television panel fabrication which utilizes electromagnetic induction (EMI) heating for certain process steps. As is readily apparent, electromagnetic induction heating makes any electronic devices, e.g., triangulation ranging sensors, in the area especially susceptible to EMI.
The CyberOptics PRS series sensors noted herein avoid such heating problems by remotely locating most electronics. However, the detector used in such PRS-type sensors is located in the sensor head and is, therefore, especially sensitive to EMI.
Another known sensor which goes a step further in removing its electronics from the sensor head is the Optech Fiberscan manufactured by Optical Technologies, Inc. of Herndon, VA. In these Fiberscan sensors, the detector is remotely located and the light spot is relayed to it using a linear array of optical fibers. However, these Fiberscan sensors do not use a remote source of light, using instead a source of light that is located in the sensor head, thereby making it susceptible both to EMI and ambient heat.
In view of the preceding limitations of known triangulation ranging sensors, it is an object of the present invention to provide non-contacting methods and apparatus for determining the curvature of an object surface which are resistant to heat, vibrations, and EMI.